Metallocene catalyst compounds that are low comonomer incorporators typically show very low activity. In addition, polymer molecules of low comonomer content are, in general, relatively more crystalline and have a high melting temperature, which is disadvantageous in applications such as film where softness or clarity is desired. However, polymers produced by these low incorporators have been shown to be useful in preparing polymer blends. Such blends, for example, include two polymers with each polymer having the same molecular weight, but different comonomer content. Typically these blends have improved mechanical, physical and/or chemical properties and produce articles of manufacture with superior properties. For example, polymer blends prepared with broad bimodal composition distributions consisting of two narrow populations are known to have superior properties such as high tear values in films.
U.S. Pat. No. 5,382,630 to Stehling et al. discloses linear ethylene interpolymer blends made from components that can have the same molecular weight but different comonomer contents, or the same comonomer contents but different molecular weights, or comonomer contents that increase with molecular weight.
U.S. Pat. No. 6,051,728 to Resconi et al. discloses a class of metallocene compounds having two substituted cyclopentadienyl rings bridged by an alkylidene group wherein the groups can be indenyl groups. The metallocenes are characterized by (i) the cyclopentadienyl groups are substituted at the 3-position with a substituent other than hydrogen while the 2-position bears a hydrogen or is part of a condensed benzene ring; (ii) the bridge is a substituted-methylene bridge; and (iii) the cyclopentadienyl groups are identically substituted. These catalyst compositions are utilized in olefin polymerization and particularly in propylene polymerization. However, catalysts with short bridges, such as methylene bridges, particularly those with indenyl or flourenyl ligands, are generally known to have very high comonomer incorporation.
Angew. Chem. 101 (1989) No. 11, 1536 Herrman, Rohrmann et al. discusses four atom bridged bis-indenyl metallocenes.
There is a need in the industry for the ability to identify polymerization catalyst compounds having low comonomer incorporation and enhanced activity to produce polymers and polymer blends with enhanced properties.
In addition, there is a need in the industry for catalyst systems to produce polymers with enhanced properties imparted by low incorporator catalysts to produce polymers, utilizing more than one catalyst in a single reactor.